MULTIPLE SYSTEMS OF CATEGORIZATION IN HUMANS AND NONHUMAN PRIMATES Category learning is a basic cognitive function for humans and nonhuman animals and a focus in human and animal research. The proposed research would integrate these research traditions in many ways. It would 1) create paradigms that let human and animal categorization be closely compared; 2) extend to the primate literature constructive developments in the human literature; 3) describe the category-learning system from which that of humans evolved; 4) trace the development of categorization across a time depth of primate phylogeny; 5) evaluate the multiple-systems structure of primate categorization; 6) evaluate the extent to which primates use an explicit, rule-based categorization system; 7) ask whether primates dimensionalize their categories and stimuli as humans do; 8) explore explicit cognition by primates in the domain of dimensional rules and hypotheses; and 9) open a new window on primates' declarative cognition. The research will also provide some of the first studies evaluating animals' capacity to sustain categorization in the absence of immediate, trial-by-trial feedback, to self-instruct, to bridge their trial-to-trial performance with explicit rules, and to declare the rule they are using. These capacities are critical aspects of humans' on-line cognition. The research also engages and advances the debate about reinforcement as the binding force in animal learning. The research will advance the understanding of human categorization by including the first studies that 1) analyze the interaction and competition between explicit and implicit systems of categorization; and 2) evaluate cross-modal implicit and explicit category learning. The research will also challenge the casual theoretical link made between rule-based cognition and verbalization/language, by studying nonverbal species in which that link is broken. The research will foster a dialog among comparative, cognitive, and neuroscience researchers, by allowing the comparison of human and animal categorization abilities and limitations to be correlated with the differential development of the brain systems that serve category learning.